Comparative research is proposed on mechanisms of vocal production. Two foci are the production of stereotypic phonological units and the role of the vocal tract in production. Studies of production are integrated with learning and perception experiments. Avian vocal behavior, an established model for certain problems in speech, is used as a paradigm. Songbirds learn to sing by reference to auditory models. Many species, however, accept only a narrow range of sounds as models, resulting in a limited, species-typical, and highly stereotypic repertoire of phonological elements. Physiological experiments will determine the nature of the correspondence between stereotypy in the acoustic structure of phonological units and the similarity of motor programs underlying their production. Individuals will be tutored during song acquisition with models of known acoustic and motor similarities to determine how acoustic features and underlying motor patterns influence vocal development. Perceptual discrimination experiments will determine how adults categorize acoustic and motor similarities of specific phonological units. General acoustic and functional morphological properties of the vocal tract and their role in vocal production will be experimentally analyzed. Anatomical data and static acoustic properties of avian vocal tracts will be obtained. Kinematic data will be used to describe dynamic changes in vocal tract morphology during vocalization and to correlate changes in morphology with time-varying acoustic properties of vocal behavior. X-ray cinematography will provide comparable data for internal features. Biomechanical models will be used to develop functional anatomical hypotheses that can be tested with kinematic data. Electromyographic data will be used to measure patterns of muscle activity that control vocal tract motions. Experiments will determine if acoustic attributes of signals mediated by the vocal tract, specifically the tonal structure of sounds, are influenced by learning. Perception of vocal tract acoustic attributes will also be tested. The development of coordination in complex systems that produce patterned behavior, such as speech, is the focus of much research in the neurobiology of motor control. The results of the proposed studies will have implications for understanding the relationships between learning, development, and perception in vocal production, as well as the of complex motor systems in general.